docs: def get_info(self, sm_id):
          """
          Extract a CompositionInfo instance containing the single
          model of index `sm_id`.
          """
          sm = self.source_models[sm_id]
          num_samples = sm.samples if self.num_samples else 0
          return self.__class__(
              self.gsim_lt, self.seed, num_samples, [sm], self.tot_weight)